Optical properties of undoped, Eu3+ doped and Li+ co-doped Y2Hf2O7 nanoparticles and polymer nanocomposite films

Abstract

Desirable phosphors for lighting, scintillation and composite films must have good light absorption properties, high concentration quenching, high quantum efficiency, a narrow color emission, and so forth. In this work, we first show that undoped yttrium hafnate Y₂Hf₂O₇ (YHO) nanoparticles (NPs) display dual blue and red bands after excitation using 330 nm light. Based on density functional theory (DFT) calculations, these two emission bands are correlated with the defect states arising in the band-gap region of YHO owing to the presence of neutral and charged oxygen defects. Once doped with Eu³⁺ ions (YHOE), the YHO NPs show a bright red emission, a long excited state lifetime and stable color coordinates upon near-UV and X-ray excitation. Concentration quenching is active when Eu³⁺ doping reaches 10 mol% with a critical distance of ∼4.43 Å. This phenomenon indicates a high Eu³⁺ solubility within the YHO host and the absence of Eu³⁺ clusters. More importantly, the optical performance of the YHOE NPs has been further improved by lithium co-doping. The origin of the emission, structural stability, and role of Li⁺-co-doping are explored both experimentally and theoretically. DFT calculation results demonstrate that Li⁺-co-doping increases the covalent character of the Eu³⁺–O²⁻ bonding in the EuO₈ polyhedra. Furthermore, the YHOE NPs have been dispersed into polyvinyl alcohol (PVA) to make transparent nanocomposite films, which show strong red emission under excitation at 270 and 393 nm. Overall, we demonstrate that the YHO NPs with Eu³⁺ and (Eu³⁺/Li⁺) doping have a high emission intensity and quantum efficiency under UV and X-ray excitation, which makes them suitable for use as phosphors, scintillators and transparent films for lighting, imaging and detection applications.